Numerous applications exist in oil production and soil remediation where it is desired to uniformly heat thick sections of the earth using thermal conduction. In the case of oil production, there exist enormous worldwide deposits of oil shale, tar sands, lipid coals, and oil-bearing diatomite where uniform heating of the hydrocarbonaceous deposit by thermal conduction can be used to recover hydrocarbons as liquids or vapor. The thickness of the deposits can be hundreds of feet thick, and lie beneath overburden hundreds of feet thick. In the case of soil remediation, uniform heating of the soil by thermal conduction can vaporize contaminants and drive them to production wells, or even destroy the contaminants in situ. Here, the contamination can extend from the soil surface down hundreds of feet.
Electric heat can be used for uniform heating of thick earth formations by thermal conduction, as is well known in the art. However, electric heating is generally expensive due to a higher per-BTU cost of electricity as opposed to hydrocarbon fuels. This relatively high energy cost can unfavorably affect the economics of oil recovery and soil remediation. Heat by combustion of natural gas is substantially less expensive and is therefore generally preferred to electric heat. However, it is difficult to uniformly heat thick earth formations, especially when those formations are below overburdens of hundreds of feet. This is particularly true when injection of 300 Watts/ft or more heat to the earth formation is desired. This can be the case in oil production and soil remediation heat injection applications.
Existing burner technology would result in large temperature variations between the top and bottom of the heated interval and non-uniform heating of the earth formation. Examples of burners suggested for such services include Swedish patent No. 123,137, and U.S. Pat. Nos. 2,902,270 and 3,095,031. These burners have flames within wellbores. The radiant heat source within the wellbores requires that expensive materials be used for major portions of the wellbore tubulars. With downhole gas-fired burners, the well casing adjacent to the burner becomes significantly hotter than the average well temperature, resulting in early casing and burner failures unless very expensive materials are utilized. This problem is exacerbated because the typical heating time in oil recovery applications may be two years or longer. In applications with thousands of such wells operating simultaneously (such as recovery of hydrocarbons from oil shale) the gas burners must be easy to maintain and preferably maintenance free. Further, coke formation within the fuel gas conduits would be a significant problem in operation of such burners.
U.S. Pat. No. 3,181,613 suggests utilizing an ignition propagation rod (a ceramic, glass or sintered metal rod placed within a burner tube) to extend the flame over a longer distance within a wellbore. Such a flame-holding rod aids in extending the flame down the wellbore, but results in a flame that is difficult to control in that limited degrees of freedom are available for controlling the temperature and the distribution of heat within the wellbore. Further, if combustion gases return up the wellbore, heat exchange between the combustion gases and the fuel and combustion air could result in autoignition of the combined combustion air and fuel stream.
A wellbore heater with greater control over the distribution of heat within the wellbore would be desirable. In the case of oil production from oil shale, non-uniform heating of the oil shale reservoir results in some oil shale not reaching retorting temperature, and overheating other parts of the oil shale, which negatively affects economics.
It is therefore an object of the present invention to provide a method and an apparatus to heat a formation wherein burners and controls can be located exclusively at the surface, wherein materials below the surface are not exposed to flames, and wherein heat can be delivered to the formation with improved uniformity or with a predetermined pattern.